1. Field of the Invention
This invention relates to a hydrogen-absorbing alloy for use as a negative electrode in an alkaline storage battery.
2. Description of the Prior Art
Currently, research is being carried out to develop for hydrogen-absorbing alloys for use in a battery, and which are capable of absorbing and desorbing hydrogen reversibly. Also being developed are metal hydride storage batteries which use such hydrogen-absorbing alloys for their negative electrode. Metal hydride storage batteries, when compared to conventional Ni-Cd batteries or lead acid batteries, can be lighter and have a larger capacity.
Proposed hydrogen-absorbing alloys for the negative electrode in the battery, include LaNi.sub.5, LaNi.sub.4 Co and LaNi.sub.4 Cu such as described in Japanese patent publication 59-49671, hydrogen-absorbing alloys of an Mm-Ni type (where the "Mn", a so-called "Misch Metal" is a mixture of rare earth elements, such as La, Ce, Nd, Pr, etc.) are also being developed. These alloys produce batteries that can obtain a high capacity.
The above alloys are obtained by conventional methods, which typically produce alloys with homogeneous texture. This reduces cracking within the alloy. Therefore, when using these alloys for a negative electrode in a battery, the surface area does not increase, and there is a reduced reaction area. As a result, discharge characteristics of hydrogen desorbtion for the alloys at low temperatures deteriorate. Further, the oxygen absorbing capability of these alloys is inferior, thus, making rapid charging of the battery problematic.
In typical conventional process for manufacturing the hydrogen-absorbing alloys, each component of the alloy is measured and mixed, melted in an arc furnace in an inert argon atmosphere, and cooled. Next, the alloys are held in a vacuum furnace and heat-treated at a high temperature (approximately 1000.degree. C.) for a fixed period of time. This is an "annealing" heat treatment which causes homogenization of the hydrogen-absorbing alloys.
To complete the electrode, the hydrogen-absorbing alloys are mechanically pulverized into granules. The granules are mixed into a paste with a conductive agent and a binder, such as powered polytetraflourethylene (PTFE). A coating of the paste is provided on a conductive substrate, such as a current collector comprising a punched metal plate. The current collector is then pressed to form the hydrogen-absorbing alloy electrode.